1. Field of Invention
The present invention is related to rendering images. More specifically, the present invention is related to an image rendering system and method whereby the ability to precisely render images is improved without detrimentally affecting the amount of bandwidth and memory allocation necessary for image rendering.
2. Description of Related Art
Conventionally, it is difficult to phase shift halftone dots with sub-scan precision in a process direction without causing unwanted tone variation and visual systematic error, e.g., auto moire and superposed moirxc3xa9. Conventional halftoning methods and systems adjust or warp the image data produced by an image data generator, such as a grayscale image generator, or binary image generator, to minimize moire. This warping is performing by rendering image data utilizing electronic registration, as disclosed in U.S. Pat. No. 5,732,162 to Douglas N. Curry, assigned to Xerox Corporation and incorporated herein by reference in its entirety. Such systems can use high speed dithering with a high speed digital shift-register.
Although, dithering has a systematic noise associated with it because of the number of modulation transition levels performed during rendering of each output pixel, such image rendering systems are practically implemented because they do not require an excessive bandwidth to transmit data for rendering each output pixel. An output pixel is a unit of data included in an array of output pixel data at, e.g., 300 or 400 spots per inch resolution, that represents image data and is provided to a modulator in an image rendering system to render images. An output pixel may span {fraction (1/300)}th or {fraction (1/400)}th of an inch in a fast-scan direction. There may be four to eight dithered bits within each output pixel.
Warping the image data can be utilized to compensate for distortions in the imager, but may not be enough to compensate for moire introduced by these distortions. This is because merely warping the image data to minimize the moire patterns results in offsets within the image data that have no corresponding adjustment or warp in the halftone screens used to render color image separation layers. Therefore, moire pattern minimization is conventionally improved by also warping halftone screens in a halftone screen system to correspond to the warping of the image data, as disclosed in U.S. Pat. No. 5,485,289 to Curry, assigned to Xerox corporation and incorporated herein by reference in its entirety.
Although such methods and systems phase shift halftone dots in two dimensions with minimal tone variation and systematic error, such systems require a significant amount of bandwidth for transmitting image data power levels to a print engine. This is because such systems render image data by using modulation via high-speed, analog digital-to-analog converters (DACs). As a result, these systems utilize an excessive amount of data transmission bandwidth because of the amount of data necessary to perform this modulation, i.e., bandwidth necessary to transmit approximately double the number of data bits used to modulate light source during image rendering.
Additionally, recent developments in hexagonal halftone screen phaseshifting have increased the already present need to find a feasible way to implement halftone dot phaseshifting during image rendering. See, for example, U.S. patent application Ser. No. 09/244,767, filed on Feb. 5, 1999, by Douglas N. Curry, assigned to Xerox Corporation and hereby incorporated by reference in its entirety.
FIG. 1 illustrates conventional image rendering in a fast-scan direction. As shown in FIG. 1, a light source is modulated between full-on and full-off over the span of the output pixel to render a grayscale value for the output pixel. This modulation is known as dithering. Conventionally, it is preferable that dithering is performed at the highest frequency possible in order to provide an even tone to grayscale values rendered for an output pixel. Since there are 4xc3x97 of these independently addressable bits for each output pixel, the dither rate is said to be 4xc3x97 high addressability. By modulating the light source so that light source has a duty-cycle of 50%, a grayscale value is rendered that is halfway between that provided when the light source is on 100% and when the light source is off. In this way, the duty-cycle of the light source is adjusted to provide various grayscale values for output pixels.
However, as explained above, dithering has an associated noise that results in systematic errors. As a result, use of conventional dithering can cause moire patterns in the rendered image. Additionally, because of the high frequency of modulation level transitions a significant amount of bandwidth and high speed memory is necessary to render grayscale values using electronic registration with high-addressability.
Therefore, the present invention allows implementation of phase shifting of halftone dots in conjunction with modulation of a light source using a smaller amount of bandwidth from a remote video source. Specifically, a light source is modulated with a restricted amount of data provided by a remote video source. For example, it is preferred that a light source modulator designed in accordance with the exemplary embodiments of the invention receives only four bits of data per output pixel. The benefit of restricting the number of bits is that high speed memory allocation and costs are minimized.
It is advantageous to minimize the amount of data to be transmitted while providing the maximum amount of gray values used for image rendering. The present invention provides a hybrid analog/digital laser modulator and method of modulation for rendering images. Such a modulator and method are advantageously useful in conjunction a printing system that phase shifts binary halftone dots. By utilizing the exemplary invention embodiments, implementation of phaseshifting halftone dots is both more feasible, practical to implement and useful because of resulting lowered memory allocation requirements.
Exemplary embodiments of the invention combine the use of a low noise DAC with digital shift-registers to obtain a lower bandwidth modulation that is less susceptible to systematic error. Such a modulation scheme is preferably implemented with systems which phase shift halftone dots during image rendering.
The exemplary embodiments of the invention separately provide the capability to provide higher image data processing speeds while providing a maximized number of gray levels for image rendering and minimizing the necessary amount of bandwidth consumed during image data transmission to and processing by a modulator. Such a modulator may be used in an image rendering system that implements laser modulation to perform image rendering.
The exemplary embodiments of the invention improve the efficiency of use of high speed memory by limiting a number of modulation level transitions to a single transition per output pixel. In conjunction with the limitation of transitions per rendering of each output pixel, data is provided to the modulator which indicates an intensity level to be provided by the modulator and a timing of a transition to that intensity level. In accordance with the exemplary embodiments of the invention, an intensity level may be a minimum intensity level, a maximum intensity level and at least one intermediate intensity level which is intermediate between the minimum and maximum intensity levels.
In a first exemplary embodiment of the invention, intensity levels include a minimum level, a maximum level and one intermediate intensity level that is intermediate between the minimum and maximum intensity levels.
In a second exemplary embodiment of the invention, the number of intensity levels is increased to four, thereby allowing more flexibility to render grayscale images. Thus, in the second exemplary embodiment, the intensity levels include a minimum level, a maximum level and two intermediate intensity levels that are intermediate between the minimum and maximum intensity levels.